1. Field of the Invention
The present invention in general relates to transducers, and particularly to a novel backing member therefor for absorbing acoustic radiation.
2. Description of the Prior Art
Ultrasonic transducers find use in various fields such as medical diagnosis, flaw detection, and flow measurement systems, to name a few.
A common type of transducer includes an elongated housing or holder having at one end thereof an acoustic window for transmission of acoustic energy and behind which is positioned an active transducer element. The front surface of the active element transmits and receives acoustic energy through the acoustic window and a backing mass is generally disposed behind the rear surface of the active element in order to absorb rearwardly projected acoustic energy.
The backing mass, or damping block, is in intimate contact with the rear surface of the active element and in order to provide for good acoustic coupling so as to reduce acoustic reflections, it is desirable that the backing mass have an acoustic impedance closely matching that of the transducer active element.
Various types of backing masses have been proposed including solid materials such as tungsten metal or graphite as well as mixtures of materials such as metal powder mixed with synthetic resin or metal powder mixed with an elastomeric compound. In order to insure for a good bond between the backing member and active element, the two members are adhesively connected together.
As opposed to the solid backing masses, the metal powder-filler combinations may be tailored to provide different acoustic impedances by varying the mixtures thereof. These transducers operate satisfactorily under normal operating conditions, however, they are quite unsuitable for high temperature, high pressure operations such as may be found, for example, in an acoustic flow measurement system of an atomic reactor primary or secondary loop. Prior art mixtures do not function properly above approximately 71.degree. C. (160.degree. F.), and further at elevated temperatures, the adhesive bonding material is severely affected.